The present invention relates to glass compositions, methods of forming the glass, and articles comprised of the glass. In particular, the glass compositions of this invention are comprised of silicon, oxygen, and carbon with a substantial portion of the carbon atoms being bonded to silicon atoms.
Amorphous silica is a refractory glass, however, it devitrifies readily at temperatures greater than 1100.degree. C. Devitrification refers to the ordering or crystallization of the random structures that glasses are made of. Crystallization drastically reduces one of the predominant attributes of vitreous silica, i.e., its low thermal expansion, as well as many of its other desirable properties. As a result, much research has been directed to seeking ways to increase the resistance to devitrification in silica glass compositions.
Reactions between silicon, carbon, and oxygen have been studied extensively. Some of the known reactions in a silicon, carbon, and oxygen system include oxygen combining with silicon to form silica, SiO.sub.2. At temperatures in excess of 1100.degree. C. silica begins to crystallize to form cristobalite, one of the common mineral forms of silica. Carbon can react with available silica to form crystalline silicon carbide or escape as carbon monoxide gas. Any carbon remaining as elemental carbon readily oxidizes above 600.degree. C. when exposed to air.
The thermodynamics of silicon, carbon and oxygen reactions is discussed in "The High-Temperature Oxidation, Reduction, and Volatilization Reactions of Silicon and Silicon Carbide", Gulbransen, E. A., and Jansson, S. A. Oxidation of Metals, Volume 4, Number 3, 1972. The thermodynamic analysis of Gulbransen et al. shows that at 1200.degree. C. silica and carbon should form gaseous silicon monoxide and carbon monoxide or solid silicon carbide, SiC. However, no material containing silicon, oxygen and carbon would be expected to form. Gulbransen et al. conclude that silica was not recommended for use in reducing atmospheres above 1125.degree. C. due to the formation of volatile silicon monoxide gas. Also silicon carbide was not recommended for use in oxygen containing environments where active oxidation may occur due to oxidation of the silicon carbide.
There is an opaque, black, glass that is functionally described as carbon modified vitreous silica and herein referred to as "black glass" where 1-3 percent carbon has been added to silica. The method for making black glass is disclosed by Smith et al. in U.S. Pat. No. 3,378,431. Carbonaceous organics such as carbowax are added to silica and the mixture is hot pressed at about 1200.degree. C. to form black glass. Smith, C. F., Jr. has further characterized black glass by infrared spectroscopy in "The Vibrational Spectra of High Purity and Chemically Substituted Vitreous Silicas", PhD Thesis, Alfred University, Alfred, N.Y., May 1973. Smith discloses that in addition to elemental carbon dispersed in the glass, carbon in black glass is associated with oxygen in carbonato type groups. A carbonato group is the description of a particular way that a carbon atom bonds with three oxygen atoms and has the structure, ##STR1##
The mechanical strength of black glass is similar to the strength of carbon free silica glass, however, black glass has an increased resistance to devitrification over conventional silica glass which begins to devitrify at about 1100.degree. C. while black glass begins to devitrify at about 1250.degree. C. The increased thermal stability of black glass allows it to be used at temperatures higher than vitreous silica can withstand.
In a commercially produced continuous silicon carbide ceramic fibre sold under the trademark "Nicalon", about 10 percent oxygen is introduced into the fibre to crosslink it. After crosslinking, the fibres are pyrolized and it is believed that the oxygen becomes part of the fibre as an amorphous contaminant, probably in the form of silica. The degradation behavior of such fibres after heat treatment in various environments was reported in the article "Thermal Stability of SiC Fibres (Nicalon.RTM.", Mah, T., et al., Journal of Material Science, Vol. 19, pp. 1191-1201 (1984). Mah et al. found that regardless of the environmental conditions during heat treatment, the "Nicalon" fibre strength degraded when the fibres were subjected to temperatures greater than 1000.degree. C. The fibre degradation was associated with loss of carbon monoxide from the fibres and beta-silicon carbide grain growth in the fibres.
Ceramic materials generally exhibit brittle behavior as characterized by their high strength and low fracture toughness. Fracture toughness is the resistance to crack propagation in materials. The development of ceramic composites has been investigated as a way to alleviate the brittle behavior of ceramics. "Nicalon" is an excellent ceramic fibre but it degrades at temperatures above 1200.degree. C. Integrating "Nicalon" fibres in a protective ceramic matrix having desirable mechanical properties and capable of withstanding temperatures substantially higher than 1200.degree. C., would be one way of forming an improved ceramic composite. However, from the discussion above, it is apparent that the properties of known ceramic or glass compositions, and specifically those containing silicon, oxygen and carbon, are degraded by decomposition or devitrification of the glass or ceramic at temperatures above 1100.degree. C. to 1250.degree. C.
Therefore, it is an object of this invention to form glass compositions, comprised of chemically bonded silicon, oxygen and carbon wherein a substantial portion of the carbon atoms are bonded to silicon atoms, and the glass is formed from select methyl silicone resins.
It is another object of this invention to form a translucent glass, comprising chemically bonded silicon, oxygen and carbon wherein a substantial portion of the carbon atoms are bonded to silicon atoms with up to trace amounts of elemental carbon dispersed in the glass matrix. Such glass compositions remain structurally stable and do not decompose in oxidizing or reducing atmospheres at temperatures up to at least 1600.degree. C.
Another object of this invention is a process for forming such glasses comprised of silicon, oxygen and carbon by pyrolizing select methyl silicone resins.
Still another object of this invention is the formation of such a glass comprised of silicon, oxygen and carbon into articles.